Linear array eye tracker

ABSTRACT

Improved devices, systems, and methods for sensing and tracking the position of an eye make use of the contrast between the sclera and iris to derive eye position. In many embodiments, linear photodetectors extend across the pupil, optionally also extending across the iris to the sclera. A pair of such linear photodetectors can accurately sense and measure one-dimensional positioning error and provide feedback to a one-dimensional positioning apparatus, resulting in a simple, highly linear system capable of accurate position tracking.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of, and claims the benefit ofU.S. patent application Ser. No. 09/249,912, filed Feb. 12, 1999, nowabandoned; which is a continuation application of U.S. patentapplication Ser. No. 09/063,879, filed Apr. 21, 1998, now U.S. Pat No.5,966,197 the full disclosures of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION FIELD OF THE INVENTION

The present invention is generally concerned with ophthalmic instrumentsand surgery, and more particularly relates to systems, methods, andapparatus for sensing and/or tracking the position of a human eye. Thepresent invention is particularly useful for tracking the position ofthe eye during laser eye surgery, such as photorefractive keratectomy(PRK), phototherapeutic keratectomy (PTK), laser in situ keratomileusis(LASIK), or the like. In an exemplary embodiment, the present inventionis incorporated into a laser ablation system to modify the distributionof laser energy directed at the cornea based on the sensed position ofthe eye during the laser ablation procedure.

The ability to track or follow the movement of a patient's tissue isrecognized as a highly desirable feature, particularly for use in laserdelivery systems designed to effect precision surgery in delicate oculartissue. The eye movements to be tracked include not only the voluntarymovements (which can be damped with specialized treatment), but also theinvoluntary movements which are more difficult to control in a livingpatient. In other words, even when the patient is holding “steady”fixation on a visual target, eye movement still occurs. This involuntarymotion may compromise the efficacy of some ocular surgical procedures,which generally require a rate of precision. In fact, such involuntarymovements may occur despite the “total immobilization” of the eye, assuch techniques are not fully effective in suppressing involuntary eyemotion, and are also rather uncomfortable for the patient. Automatictracking of the eye may alleviate any need for this uncomfortableimmobilization, and may offer a method for more effectivelyaccommodating differing types of eye motion. In other words, augmentingsurgery with real time eye tracking may improve the accuracy and speedwith which known laser eye surgery can be performed, and may also enablenew procedures to be carried out for the first time.

A variety of techniques have been described for tracking eye movements.One general type of eye tracking technique has been called “opticalpoint tracking.” Optical point trackers utilize various lens-likeproperties of the eye to locate optically distinguishable locations (forexample, the first, second, third, and fourth Purkinje points).Unfortunately, such optical point trackers implicitly assume that theeye moves as a rigid body. As the eye actually flexes during movement,transient relative motions of lens structure can lead to fictitiousoptical point position information. In addition, optical point trackingsystems are rather complex, and may exhibit large variability betweenindividuals.

Another class of eye tracking techniques generally involve digitalpattern recognition. These digital techniques generally require veryfast frame-rate CCD cameras and sophisticated processing algorithms. Astracking frequency response is considerably slower than updatefrequency, they tend to be relatively slow. Hence, these known digitalmethods generally require extremely fast repositioning mechanisms toleave time for complex electronic processing within an acceptable totalresponse time.

A recent promising technique for tracking eye movements takes advantageof the difference in the light scattering properties of the iris andsclera. In this technique, light is projected on to the iris/sclerainterface or limbus, and the scattered light is detected byphotodetectors to determine the boundary location. The relative positionof this boundary can then be monitored to track the position of the eye.

Unfortunately, the limbus is more a transition zone between the corneaand the sclera, rather than a sharp boundary. As a result, techniqueswhich rely on edge detection may lack the desired accuracy, and may notbe capable of tracking large amplitude movements of the eye. Anotherdisadvantage of known limbus tracking techniques is the relativecomplexity of signal processing required to effect tracking. In otherwords, when the eye moves so that the limbus is no longer in the nominalposition, effecting realignment using known tracking systems requiresfairly complex manipulations of the photodetector signal to properlyinstruct the repositioning system. These complex signal manipulationsincrease overall system complexity, and also slow the system down. Workin connection with the present invention indicates that slow trackingsystem response and less than desirable accuracies may in-part be theresult of tracking system non-linearities. While adequate trackingresponse may be possible using known “pin-point” limbus trackers withaccurately aligned photodetectors disposed precisely along the edge ofthe iris/sclera interface, providing and/or maintaining such alignmentadds additional system components and complexity, particularly in lightof the variability of eye geometry between differing patients.

In light of the above, it would be desirable to provide improved eyesensing and tracking devices, systems, and methods. It would beparticularly desirable if these enhanced techniques improved trackingresponse times and sensitivity, but without significant increases incost or complexity of the tracking mechanism. It would be particularlydesirable to provide these enhanced capabilities in a system which wasadaptable for use in laser eye surgery for accurately sensing and/ortracking a variety of patient eye movements.

SUMMARY OF THE INVENTION

The present invention provides improved devices, systems, and methodsfor tracking the position of an eye. The techniques of the presentinvention generally make use of the difference in contrast betweenfeatures of the eye (such as between the white of the eye or sclera andthe colored iris, or between the iris and the pupil) to derive theposition of the eye. In many embodiments, linear photodetectors havingan elongate sensing area extend from one feature to another across thecontrast border. Where the eye is positioned between a pair of suchlinear photodetectors, movement of the eye from one linear detectortoward the other linear detector will change the relative amounts oflight striking each linear detector. The amount of misalignment betweenthe linear detectors and the eye will be proportional to the differencein the signal output by the detectors. Therefore, this difference insignal between a pair of opposed linear photodetectors provides anexcellent feedback signal, requiring only very simple amplification foruse as an input signal for a repositioning mechanism. Such simple signalprocessing not only reduces the circuitry complexity and cost, butsignificantly enhances the speed and accuracy of tracking.

Conveniently, linear photodetectors can accurately sense and measureone-dimensional positioning error of a substantially round feature suchas the iris or pupil. The tracking systems of the present inventionoften take advantage of this one-dimensional error measurement bymeasuring light along two axial segments which cross the contrast borderand each other. This arrangement can provide accurate relative positioninformation despite the lack of a sharp contrast boundary, such as whenusing the gradual contrast boundary often found at the limbus.

In a first aspect, the invention provides a laser eye surgery system foreffecting a desired change in optical characteristics of an eye duringlateral movements of the eye in X and Y orientations. The eye has afirst feature and a second feature with a contrast border therebetween.The system comprises a first linear photodetector having an elongatedetector area. The detector area of the first linear photodetector isoriented to receive light from the first and second feature of the eyealong a first axis extending across the contrast border. A second linearphotodetector has an elongate detector area which is oriented to receivea light from the first and second feature of the eye along a second axisextending across the contrast border. The second axis is disposed at anangle relative to the first axis. A processor is coupled to the firstand second linear photodetectors. The processor calculates the lateralmovement of the eye in the X and Y orientations in response to the lightreceived from along the first and second axes. A laser is coupled to theprocessor. The laser directs a laser beam toward the eye to effect thedesired change in optical characteristics of the eye. The beam moveslaterally in response to the calculated lateral movements of the eye.

The first and second linear detectors will often sense light fromelongate areas of the eye which are at substantially perpendicularorientations relative to each other, although oblique angle alignmentsmay also be used. Optionally, each linear detector may produce a signalindicating total light received within the elongate detector area.Alternatively, each linear detector may comprise a linear array of lightsensors, so that each linear detector produces a signal indicating analignment of the contrast border along the light sensors. When measuringthe location of a round feature such as the iris or pupil, such sensorsmight indicate the axial position of the round feature by determiningthe axial location of, for example, a low light measurement region alongthe array. Alternatively, the array may be analyzed as two separate setsof sensors by comparing the total light received from the sensors on oneside of the array to the total light from sensors on the other side ofthe array.

The system will often include imaging optics associated with the firstand second linear photodetectors. Such imaging optics can independentlyimage the light from the eye on to the first and second linearphotodetectors. This allows, for example, imaging of crossing axialsegments on to the two photodetectors, with the axial segmentspreferably crossing within a pupil of the eye. The optics may bedisposed off the optical axis of the eye, preferably offset from theoptical axis by 90° so as to isolate X and Y lateral movements of theeye. In general, tracking system complexity and response time can beimproved by independently coupling the first and second linear detectorsto first and second beam scanning actuation systems, with each actuationsystem maintaining alignment between the linear detector and the eyealong the sensing axis of the associated linear detector.

In another aspect, the invention provides a laser eye surgery methodcomprising illuminating a first feature and a second feature of an eye.Light is measured from the illuminated eye with a plurality of linearphotodetectors while each linear detector is aligned to receive lightfrom adjacent the first feature, from the second feature, and fromacross a contrast border therebetween. A lateral movement of the eye isdetermined from the measured light. A laser beam is directed toward acorneal tissue of the eye to effect a desired change in opticalcharacteristics of the eye. The beam moves laterally in response to thedetermined lateral movements of the eye.

In the exemplary embodiment, the light from the illuminated eye isimaged independently by imaging optics so that first and second axialsegments of the eye are imaged on to the first and second linearphotodetectors, respectively. Ideally, the first and second axialsegments cross within a pupil of the eye, the contrast border comprisingthe pupil/iris boundary and/or the limbus.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates the tracking system of the presentinvention, in which lateral movements of the eye are measured from thedifference in light intensity measured between two pairs of bulk linearphotodetectors along two independent measurement/repositioning axes.

FIG. 1A schematically illustrates a photodiode structure for use in thesystem of FIG. 1.

FIG. 2 is a schematic side-view of a laser surgery system including thetracking system of FIG. 1 for one of the two independent axes.

FIGS. 3A and 3B and 3C and 3D illustrate a method for sensing lateraleye movements in one-dimension using a pair of coaxial linear bulkphotodetectors.

FIG. 4 schematically illustrates an alternative eye movement sensingsystem including two linear photodiode arrays, thereby providingabsolute limbus location sensing as well as relative translation fromthe sum of linear array outputs.

FIG. 5 schematically illustrates a method for measuring velocities usinglinear photodetectors.

FIGS. 6 and 7 schematically illustrate alternative linear photodetectorarrangements in which imaging optics independently image the eye towarddigital linear photodetectors so as to sense light from perpendicularaxial segments which cross within the pupil of the eye.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

The present invention is directed to devices, methods, and systems forsensing and/or tracking the position of an eye in a living body. Thetechniques of the present invention generally make use of the contrastof a recognizable large scale boundary of the eye, such as at thecornea/sclera interface (the limbus). The sensing or tracking systemsmay optionally determine the location and velocity of these boundarieswithout having to resort to digital sampling techniques. In a preferredaspect, the cornea/sclera interface position is tracked relative to aspecific axis using a pair of elongate photodetectors. By arrangingthese elongate detectors so that each extends across the relatively darkiris and beyond the limbus to the white sclera, the relative position ofthe limbus (and the pupil) can be determined.

The present invention optionally makes use of linear bulkphotodetectors. These photodetectors are capable of providing a signalwhich indicates a total illumination along an elongate light sensingarea To take advantage of the significant contrast between the scleraand iris, without having to pinpoint and track a position of a boundarybetween these large, high contrast structures, the light sensing areawill extend across (and beyond) the limbus.

The devices, systems, and methods of the present invention may findapplication in a variety of settings. For example, the eye positionsensing techniques of the present invention may be used for clinical oracademic studies of both saccadic and voluntary eye movements. Thesetechniques and structures will find their most immediate application inaugmenting laser eye surgery. More specifically, the tracking systems ofthe present invention may be used to maintain alignment between atherapeutic laser beam and an eye to enhance the accuracy of laser eyesurgery directed at reshaping of the cornea. Alternatively, the pairs oflinear photodetectors may be used without tracking to interrupt such alaser photoablation procedure whenever the eye moves beyond anacceptable aligned range. Regardless, the paired linear bulkphotodetectors of the sensing/tracking system of the present inventionoffer enhanced system response times over a broad range of eye motionamplitudes.

Referring now to FIG. 1, a tracking system 10 is used to track lateralmovements of an eye E using a series of linear bulk photodetectors 12.Detectors 12 are arranged in coaxial pairs, with signals from thedetectors compared by a processor 14, the processor manipulating thedetector signals to direct a repositioning mechanism 16. Repositioningsystem 16 will then alter alignment between eye E and detectors 12 basedon the signals from the processor.

Detectors 12 each have an elongate light sensing area, the detectorsgenerally being radially oriented. While detectors 12 are illustratedsuperimposed on eye E in the schematic of FIG. 1, it should beunderstood that the detectors will often sense a position of eye E basedon an image of the eye. Hence, descriptions of the relative positions ofdetectors 12 relative to the structures and features of eye E willoften, in practice, be carried out using an image of the eye. Forexample, eye E includes a sclera S and an iris I with a limbus Ldefining the border therebetween. Photodiodes 12 are disposed “across”limbus L to extend from iris I to sclera S, so that each bulk detectormeasures light from both the substantially white, relatively brightsclera, and from the much darker iris. However, it should be understoodthat the detector structures may be at some distance from the eye, sothat the detectors actually extend across an image of the eye. The imageof the eye will often be produced by an optical train between the eyeand the detectors. Alternatively, the photodiodes may be mounted on aspectacle frame near the eye and oriented directly across thesclera/iris interface.

Linear detectors 12 will typically comprise elongate siliconphotodiodes. Silicon photodiodes typically have time constants of tensof picoseconds. As a result, the sampling rate will often be limited bythe exposure time. More specifically, sampling rate is inversely relatedto exposure time, so that the shorter the exposure time, the higher thesampling rate.

The spectral response for silicon photodiodes centers in the nearinfrared (typically around about 750 μm). These detectors are generallysensitive to light throughout a fairly broad spectrum, providing atleast about fifty percent sensitivity throughout the range from 450 μmto 950 μm. The preferred illumination source will ideally include asignificant output within this range when silicon photodiode detectorsare used. Alternatively, detectors 12 may sense light anywherethroughout the range of about 350 to 1100 μm, either by making use oflower sensitivities, using alternative diode structures, or the like.

An exemplary silicon photodiode structure is illustrated in FIG. 1A.Linear detector 12 includes an array of detector elements 13. Detectorelements 13 are wider laterally (relative to the detector axis) thantheir axial length. This increases the overall detection area whilepreserving axial resolution. Hence, this structure provides increasedaxial signal to noise performance at the expense of resolution along anunused transverse sensing orientation.

Processors 14 will generally compare the signals produced by a pair ofopposed detectors 12. The detectors will be long enough to measurelateral movements of eye E along one dimension, and will be much longerthan their width. Processor 14 a measures a position of iris I of eye Ealong an axis Y by comparing a signal produced by a first detector 12 ato the signal produced by a second detector 12 b. When eye E movesupward, the amount of sclera S adjacent first detector 12 a willdecrease, while the amount of the sclera adjacent the second detector 12b will increase. Conversely, the darker iris will increasingly beexposed to first detector 12 a, and will have a decreasing exposure tosecond detector 12 b. As a result, the total illumination signalproduced by first detector 12 a will decrease, while the signal producedby the second detector 12 b will increase. By comparing these signals,processor 14 a can sense that eye E has moved in the positive Ydirection, and can also measure the amount and velocity of that movementbased on the quantitative difference in signals, and by the rate ofchange of this difference, respectively.

Processors 14 may optionally comprise relatively simple analog circuits,or may alternatively include one or more analog-to-digital converterscoupled to a digital processor. Use of an analog circuit may bepreferred to enhance system response, particularly when repositioningmechanism 16 is adapted for use with an analog input signal.

Repositioning mechanism 16 will generally effect realignment betweendetectors 12 and eye E based on the positioning signal from processor14. To separate the one-dimensional feedback loops along X and Y axes asillustrated in FIG. 1, positioning mechanism 16 a attached to processor14 a will preferably affect only the alignment along axis Y. A varietyof mechanisms may be used to provide such one-dimensional repositioning.For example, repositioning mechanism 16 a may translate the spectacleframe supporting detectors 12 along the axis. Alternatively,repositioning mechanism 16 may pivot a mirror to effect realignmentbetween an image of eye E and detectors 12. Where processor 14 providesan analog signal to repositioning mechanism 16, the repositioningmechanism will often include an analog electromechanical actuator suchas a voice coil motor, or the like. Where processor 14 provides adigital signal to the repositioning mechanism, digital electromechanicalactuators, such as stepper motors, may instead be used.

FIG. 2 illustrates a system 20 for selectively photoablating cornealtissues so as to effect reshaping of the cornea. Laser ablation system20 incorporates the elements of tracking system 10 of FIG. 1. Laserablation system 20 also includes a laser 22 which produces a laser beam24. Laser beam 24 and linear detectors 12 are aligned relative to eye Eby repositioning mechanism 16. In this embodiment, repositioningmechanism 16 makes use of a pivoting mirror 26 to alter a position of animage of eye E upon linear detectors 12. In other words, a limbus imageL′ superimposed on detectors 12 is aligned relative to the detectors bypivoting mirror 26 as shown. An optical train (not shown) may beincluded in positioning system 16 to image the eye, and to direct laserbeam 24.

Imaging and sensing can be enhanced by illuminating eye E with lightenergy appropriate for measurement by detectors 12, as described above.Such illumination can be provided by oblique illuminators 28. Theportions of tracking system illustrated in FIG. 2 will generallymaintain alignment between laser beam 24 and eye E only along axis X. Asecond pair of detectors 12 coupled to an independent processor 14 and asubstantially independent repositioning mechanism 16 can be used totrack the eye during movements into and out of the plane of the drawing.An improved tracking system according to the invention usingrepositioning mirrors might be incorporated into a laser eye surgerysystem commercially available from VISX, Incorporated of Santa Clara,Calif., under the trademark STAR™ or STAR S2™.

A change in relative signals from linear detectors 12 can be understoodwith reference to FIGS. 3A and 3B. Each of detectors 12 defines anelongate light sensing area 30 having an inner end 32 and an outer end34. Inner ends 32 are generally aligned with iris I, while outer ends 34extend out to the surrounding sclera. As a result, detectors 12 extendacross limbus L and will sense a light in part from the relatively darkiris I, and in part from the significantly brighter sclera.

Detectors 12 will generally operate in pairs to sense the relativeposition of iris I. First detector 12 a and second detector 12 b arealigned coaxially along axis X. Qualitatively, when iris I moves to theright relative to detectors 12 (as illustrated in FIG. 3A, or whenmoving from point B to point C in FIG. 3B), more of the bright sclera isexposed to first detector 12 a, thereby increasing its output signal.Conversely, more of second detector 12 b is blanketed by the dark iris,thereby decreasing its signal. However, where iris I movesperpendicularly relative to axis X (such as from point A to point B asillustrated in FIG. 3B), the signal strength from both first detector 12a and second detector 12 b will decrease by about the same amount.Hence, by comparing the signal from first detector 12 a relative to thesignal from second detector 12 b, a pair of detectors can be used toindicate movement of iris I along axis X independently of any motionalong a transverse axis Y.

Quantitatively, the signal from second detector 12 b (and for each ofthe detectors) will be: S = ∫₀^(l)I(x)  x

in which l is the length of second detector 12 b, and I(x) is theintensity at a position x along length l. As an example, FIG. 3Cillustrates an intensity profile comprising a step function with twodifferent constant values: an arbitrary low intensity such as I=100within iris I, and an arbitrary high intensity such as I=200 along thesclera. If we assume that half the length of second detector 12 b isinitially aligned with the iris and half is aligned with the sclera(l_(o)=l/2), the signal S is then given by:${s = {\int_{0}^{l/2}I}},\quad {{{x} + {\int_{v}^{l}{2I_{2}\quad {x}}}} = {\frac{1}{2}( {I_{1} + I_{2}} )}}$

As described above, when iris I moves toward second detector 12 b,signal S will decrease. More specifically, where iris I moves to theright by Δl so that the limbus moves from l/2 to l′, the signal fromsecond detector 12 b will decrease by: $\begin{matrix}{{\Delta \quad S} = \quad {S - {\int_{o}^{r}{I_{1}\quad {x}}} + {\int_{r}^{l}{I_{2}\quad {x}}}}} \\{= \quad {{\int_{lo}^{r}{I_{1}\quad {x}}} - {\int_{lo}^{r}{I_{2}\quad {x}}}}}\end{matrix}$

in which l′ is the new position of our theoretical limbus along seconddetector 12 b (l′=l_(o)+Δl), while I₁ and I₂ are the intensities alongthe iris and sclera, respectively. Using our constant I₁ and I₂ from ourstep function example, we now have an intensity distribution I(x) asillustrated in FIG. 3D, giving us a total change in signal ΔS asfollows:

ΔS=(I ₁ −I ₂)Δl

in which (I₁−I₂) is the contrast between the iris and the sclera.(200−100=100 in our example).

Another way to think of the integral which gives us the signal S fromour bulk photodetector is to look at it as a moving average of the lightintensity along a slit. Advantageously, the tracking system compares theaverage light from slits which extend well beyond the gradual transitionin contrast which actually occurs at limbus L, as illustrated by thebroken line in FIGS. 3C and 3D. In contrast to the irregular variationsalong this transition, the average illumination through the opposedslits will vary smoothly when iris I moves relative to the detectors.For relatively small changes in alignment and relatively small contrastvariations, the displacement is proportional to the change in signal.

Velocity measurements can be made quite accurately by monitoring a rateof change of the position along the X axis. The accuracy for suchvelocity measurements is a function of the ratio between the contrastand the noise from detectors 12. More specifically, velocities may becalculated as a rate of change of an edge signal 35, although the edgeneed not be sharp. A moment integral can be obtained from signal samplestaken before a time interval and after the time interval. The differencein signal divided by the time interval will indicate velocity.

Good performance signal to noise (S/N) performance will provide a moreaccurate moment, thereby giving better velocity measurements. The betterthe S/N performance, the less likely a noise spike will be inadvertentlyinterpreted as a movement of the eye. In other words, if there is toomuch noise, velocity measurements become difficult because the edgeposition becomes ill defined, and the moment will have a large standarderror. Averaging of the data can help to improve the S/N performance tomore accurately calculate a fixed or slow moving edge, but sequentialsignal averaging may reduce the maximum measurable velocity.

Referring now to FIG. 4, an alternative sensing system 30 uses a pair oflinear array photodiodes 32 a, 32 b. Such a linear array can giveadditional spacial information. Specifically, the digital nature of alinear array provides absolute edge location, rather than just relativemeasurements of the iris position. The accuracy of this absoluteposition sensing system will depend on the pixel dimensions of thelinear array, as well as on classical optical constraints such as fieldof view, magnification, and the like.

The spacial information provided by linear arrays 32 is essentially thesame as a single line of video. Advantageously, a single line pixelarray avoids the limitations of standard video input, including the slowCCD refresh rates, and the like. This may provide sampling ratessignificantly higher than the typical video refresh rates of 30 or 60Hz, and preferably as high or higher than high video refresh rates ofabout 120 Hz.

Currently available linear array photo diodes often include arrays of256, 512, or 1,024 pixels. For a view field of 25 mm, the resolution ofa 1,024 linear array photodiode is 24 μm. The dimension of each arrayelement is about 2.5 μm wide by 25 μm long along the axis of the array,thereby providing quite good axial resolution. The wider dimensiongenerally helps enhance sensitivity of the array.

Advantageously, the output from each element of linear arrays 32 can besummed to provide the same information available from a bulk detector,as described above. Therefore, so long as first array 32 a and secondarray 32 b cross limbus L at radially separated positions, the sum ofthe signals from these two linear arrays can be compared to determinethe relative position of iris I along axis X between the arrays. Inother words, in addition to the absolute edge position informationprovided by the array, pairs of linear photodiode arrays can be used asbulk photodetectors to measure the relative movement of iris I from amidline M bifurcating the arrays. Therefore, multiple pairs of arraysmay be used in some applications.

The sensing and tracking systems of the present invention have generallybeen described with reference to movement along a single axis betweenpairs of detectors. As described with reference to FIG. 1, these systemswill often include a second pair of detectors for sensing and/ortracking movements transverse to the sensing axis of the first pair.While such tracking may be enhanced by maintaining an orthogonalrelationship between these two sensing axes, eyelids or otherobstructions may be avoided by placing the pairs at oblique angles.

An alternative eye movement sensing system 40 is schematicallyillustrated in FIGS. 6 and 7. In this embodiment, eye E is imaged on tofirst and second digital linear photodetector arrays 42 a, 42 b, byfirst and second imaging optics 44 a, 44 b, respectively. The imagingoptics, here illustrated schematically as a single lens, may optionallybe disposed off the optical axis of the eye, with the optical paths ofthe imaging optics preferably being offset from each other by an angle46 of about 90° relative to the optical axis of the eye. This allowseach linear photodetector to independently sense movement in the X or Yorientation, as described above.

As illustrated in FIG. 7, the detector areas of the linearphotodetectors 42 a, 42 b, receive light from the dark pupil, from thecolored iris, and from the light sclera of eye E. In general, thedetector areas receive light from two features of the eye defining acontrast border, with the linear detector preferably measuring lightfrom along an axial segment extending beyond a first feature to asurrounding second feature on both ends. Where the first featurecomprises a substantially circular structure of the eye such as a pupiland/or iris, movement of the eye along the axis of the axial segment maybe analyzed by separating the signals from the individual sensors of thedetector array into, for example, two sets, with the signals from oneside of the array compared to signals to the other side of the array todetermine movement of the eye as described hereinabove. Alternatively,the signals from the sensors along the array may be analyzed todetermine the center of the relatively dark pupil and/or iris along theaxial segment.

While the present invention has been described in some detail, by way ofillustration and for clarity of understanding, a variety of changes,modifications, and adaptations will be obvious to those who skill in theart. For example, horizontal and vertical movements of the eye may betracked by selectively comparing signals from three linear photodiodes,in which a processor treats each of the photodiodes as an element of twopairs. Hence, the scope of the present invention is limited solely bythe appended claims.

What is claimed is:
 1. A laser eye surgery system for effecting adesired change in optical characteristics of an eye during lateralmovements of the eye in X and Y orientations, the eye having a firstfeature and a second feature with a contrast border therebetween, thesystem comprising: a first linear photodetector having an elongatedetector area, the detector area of the first linear photodetectororiented to receive light from the first and second feature of the eyealong a first axis extending across the contrast border; and a secondlinear photodetector having an elongate detector area, the detector areaof the second linear photodetector oriented to receive light from thefirst and second feature of the eye along a second axis extending acrossthe contrast border, the second axis disposed at an angle relative tothe first axis; a processor coupled to the first and second linearphotodetectors, the processor calculating the lateral movements of theeye in the X and Y orientations in response to the light received fromalong the first and second axes; and a laser coupled to the processor,the laser directing a laser beam toward the eye to effect the desiredchange in optical characteristics of the eye, the beam moving laterallyin response to the calculated lateral movements of the eye.
 2. Thesystem of claim 1, wherein the elongate detector areas of the first andsecond linear detectors are substantially perpendicular.
 3. The systemof claim 1, wherein each linear detector produces a signal indicatingtotal light within the elongate detector area.
 4. The system of claim 1,wherein each linear detector comprises a linear array including aplurality of light sensors within the elongate detector area, eachlinear detector producing a signal indicating an alignment of thecontrast border along the light sensors of the linear array.
 5. Thesystem of claim 1, further comprising first imaging optics and secondimaging optics, the imaging optics in optical paths of the light fromthe eye, the first imaging optics imaging the light from along the firstaxis onto the first linear photodetector, the second imaging opticsimaging the light from along the second axis onto the second linearphotodetector.
 6. The system of claim 5, wherein the optics image firstand second axial segments of the eye onto the first and second linearphotodetectors, respectively, and wherein the first and second axialsegments cross.
 7. The system of claim 6, wherein the axial segmentscross within a pupil of the eye, the contrast border comprising at leastone of the group comprising a limbus of the eye and a border between aniris and a pupil of the eye.
 8. The system of claim 1, furthercomprising first and second beam scanning actuation systems coupled tothe first and second linear detectors by the processor, respectively,each actuation system maintaining alignment between the linear detectorand the eye along the axis of an associated linear detector.
 9. A lasereye surgery method comprising: illuminating a first feature and a secondfeature of an eye; measuring light from the illuminated eye with aplurality of linear photodetectors while each linear detector is alignedto receive light from the first feature, from the second feature, andfrom across a contrast border therebetween; determining lateralmovements of the eye from the measured light; directing a laser beamtoward a corneal tissue of the eye to effect a desired change in opticalcharacteristics of the eye; and moving the beam laterally in response tothe lateral movements of the eye.
 10. The laser eye surgery method ofclaim 9, further comprising imaging the light from the illuminated eyeso that first and second axial segments of the eye are imaged onto thefirst and second linear photodetectors, respectively, wherein the firstand second axial segments cross within a pupil of the eye.